Impressive line of military fortifications on the Franco-German border built in the years prior to 1940, designed to force the Germans to repeat the Schlieffen Plan of 1914 attack France through Belgium, if at all.
And, indeed, this is precisely what the Germans had planned to do, and the French had planned to stop. And all would have been well for the French, if a German idiot flying out to brief a forward commander hadn't essentially sent a copy of the invasion plans to French high command, forcing the Germans to come up with a new plan.
They did. It involved the Ardennes, which was the place the French hadn't bothered to seriously defend.
(Of course, the French commanders weren't exactly blameless. In French army maneouvres, 1938, the attacking commander used Rundtstedt's plan almost exactly and managed to break through with a quarter of his forces).
Frenchie#2: Oui, let us build a huge defensive wall with cannons and guns that only face one direction!
Frenchie#1: Brilliant! uhh.. what is the German army doing behind our wall?
Frenchie#2: Damn cheaters, they went around! No fair! Time out! wahhhhhhhhhh!
GOD: hehehehe, silly french people...